Production tubing for oil wells made of a composite material of continuous carbon fibre

ABSTRACT

Production tubing for oil wells comprising a plurality of cylindrical elements made of a composite material consisting of continuous carbon fibres impregnated with a thermosetting epoxy resin, joined adjacently to each other by means of a male/female connection system, wherein the male connection is present on one end of the cylindrical element and the female connection is present on the other end of the cylindrical element.

The present invention relates to a production tubing for oil wells made of a composite material based on continuous carbon fibre and an epoxy resin.

As is known, during the preparation phase of an oil well, the continuous reinforcement of its walls is necessary with the gradual deepening of the well, to prevent it from collapsing. The reinforcement structure, called casing, is substantially composed of a metallic cylindrical body inserted in the hole and adhering to the walls of the well by means of concrete.

When the well enters into production, the oil is recovered by means of a specific pipe, known as production tubing. This is a steel pipe which is inserted into the well until it reaches the level of the reservoir. At the well bottom, the tubing is fixed by means of a combined hydraulic and mechanic sealing system—called packer—which forces the oil to rise to the surface from inside the tubing without touching the walls of the casing.

This technical solution is governed by the fact that the oil, as it is extracted from the reservoir, not only contains acid substances, essentially based on sulfur such as H₂S and mercaptans, but also entrains solids, such as sand and rock residues, which could have a strong corrosive and erosive effect on the walls of the casing, damaging them and jeopardizing the functionality and operative life of the well. By flowing in the tubing, on the other hand, the corrosive/erosive effect of the oil is concentrated inside the latter and may consequently be easier to manage, for example by replacing a damaged part of the tubing or completely replacing the tubing.

The demand for steel in the construction of tubings has grown to extremely high levels, consequently causing an increase in its price, above all for tubings constructed with special steels with a high chromium and/or nickel content, necessary in reservoirs containing fluids (oil and associated gas, in addition to water layers) with high corrosive and erosive characteristics.

The reduced availability of steel and consequent price increase have led towards a search for alternative technological solutions. The Applicant, for example, has found that it is possible to prepare tubings for oil wells consisting of composite material based on carbon fibre and an epoxy resin. The application of these materials, in fact, allows the production of tubings with mechanical performances comparable with those of special chromium/nickel steels or alloys based on aluminium and magnesium but with an even higher resistance to corrosive agents based on H₂S, CO₂ and H₂O.

Furthermore, the improvements that this new technology can offer, in addition to the corrosion resistance mentioned above, are the low tendency towards the depositing of asphaltenes, the very low elasticity to thermal jumps and the much lower specific weight with respect to steel. The extremely low elasticity to thermal jumps allows a reduction in the loads at the well head and on the packer during the productive life of the well, for example in production interruptions and recoveries. Finally, the high resistance of carbon fibres, both mechanical and also to corrosion, allows the tubings to be reused various times, after the non-destructive inspections conventionally applied in the ordinary maintenance of the same.

An object of the present invention therefore relates to a production tubing for oil wells comprising a plurality of cylindrical elements, made of a composite material consisting of continuous carbon fibres impregnated with a thermosetting epoxy resin, joined adjacently to each other by means of a male/female connection system, wherein the male connection is present on one end of the cylindrical element and the female connection is present on the other end of the cylindrical element.

The cylindrical element can be obtained according to technologies known to experts in the field. It can be obtained, for example, by winding a bundle of continuous carbon fibres impregnated with the components of the epoxy resin, around a cylindrical core rotating around its own axis. The bundle of fibres is in turn removed from a bobbin which runs with an alternating movement on a horizontal guide parallel to the rotation axis of the cylindrical core. The running rate of the bobbin on the horizontal guide with an alternating movement can be regular and constant or it can be variable, for example with a velocity which tends to decrease near the ends of the core, in one and/or other direction.

At the end of the winding phase and hardening phase of the epoxy resin, the cylindrical element is recovered by removing it from the central core.

The male and female connections are preferably made of steel, for example chromium steel, more preferably steel with 18% of chromium. The connections are generally applied to the cylindrical element during the winding phase of the bundle of fibres onto the central core, covering both the cylindrical element and a part dedicated to the welding (fixing) of both male and female connections, with the carbon fibre impregnated with epoxy resin.

Alternatively, the male and female connections can be obtained with the same composite material as the cylindrical element, applying to the ends of the central core, two forms which represent the connections with the respective threadings.

Epoxy resin is a known material available on the market. Information on epoxy resin is available in scientific literature and on the network. According to the present invention, the epoxy resin can be used without additives or it can be charged with reinforcing material, for example with a mineral filler in powder form, such as ceramic, mica, silica, calcium carbonate, zirconium or glass and/or carbon microfibres, etc.

The cylindrical element thus prepared has a length varying from 8 to 12 metres and an internal diameter ranging from 8 to 30 cm. The thickness of the cylindrical element depends on the length of the final tubing or depth of the well, but generally ranges from 0.5 to 3 cm. 

1. Production tubing for oil wells comprising a plurality of cylindrical elements made of a composite material consisting of continuous carbon fibres impregnated with a thermosetting epoxy resin, joined adjacently to each other by means of a male/female connection system, wherein the male connection is present on one end of the cylindrical element and the female connection is present on the other end of the cylindrical element.
 2. The tubing according to claim 1, wherein both the male and female connections are made of steel.
 3. The tubing according to claim 1, wherein both the male and female connections are made of the same composite material as the cylindrical elements.
 4. The tubing according to any of the previous claims, wherein a mineral filler in powder form, such as ceramic, mica, silica, zirconium, calcium carbonate, or glass and/or carbon microfibres, is added to the epoxy resin. 